Generally, an intake-air system of an engine requires a passage for which fresh air can enter into the engine and to optimize the engine by decreasing intake noise and resistance.
In order to suppress the intake noise of the intake-air system, either a narrow cross-sectional area for the intake passage or a large air cleaner box has been used. Alternatively, the cross-sectional area of the intake passage has been expanded to reduce the intake resistance of the intake-air system.
Thus, a voluminous air cleaner box has been required in order to suppress the intake noise, and the cross-sectional area of the intake passage must be appropriately formed to minimize the intake resistance, thereby optimizing the function of the engine intake-air system. However, since the engine room is limited in space, it is difficult to find enough space to mount a large air cleaner box.